Colorectal cancer (CRC) is the third most commonly diagnosed cancer in males and the second in females worldwide, with an estimated 1.4 million cases and 693,900 deaths occurring in 2012 [1]. In Singapore, CRC has become the most frequent cancer with a total of 9,324 new cases diagnosed from 2010-2014 [2]. Current routine screening of CRC uses white light reflectance (WLR) colonoscopy which may reduce CRC incidence and mortality [3]. However, some individuals are still diagnosed with CRC despite recent colonoscopy [3]. This is probably because conventional WLR colonoscopy heavily relies on the visualization of gross mucosal features associated with neoplastic transformation [4]. Subtle tissue changes may not be apparent, limiting its diagnostic accuracy. Consequently, existing diagnostic guidelines recommend extensive but random biopsy samplings during colonoscopic inspections of patients [5], followed by the microscopic examination which is highly subjective and depends heavily on the experiences of the pathologists. Overall, the current approach for colonic tissue diagnosis is clinically labor intensive and a burden to the patients. There is a need to develop advanced optical diagnostic techniques for objective diagnosis and characterization of colonic tissue.
In the past few decades, polarized light imaging/spectroscopy has been comprehensively investigated for tissue diagnosis [6-17]. Polarized light implementation offers several compelling advantages: (1) surface and beneath-the-surface detection of biological tissue taken from the tissue depolarization [12, 16]; (2) tissue anisotropy analyzed through the tissue diattenuation and retardance [14, 15]; (3) enhanced tissue diagnosis through the combination of complementary depolarization, diattenuation and retardance of the tissues [8, 10]. Among the various polarized light imaging/spectroscopy techniques developed [6-15, 18], Mueller Matrix polarimetry is capable of measuring the complete polarimetric transfer function [6-10], known as Mueller matrix, of the bulk biological tissues which are optically inhomogeneous, birefringent, and absorbing media [19]. Currently, biomedical Mueller Matrix polarimetry is mostly centered on the use of short visible wavelengths of illumination light that has a limited penetration depth and cannot detect lesions in deeper areas [8-10]. The near-infrared (NIR) light, on the other hand, penetrates much deeper into the tissue, and it is well-suited for deep tissue diagnosis [13, 20-22]. Further, the reported Mueller Matrix polarimetries are acquiring either the images [7-9] or the optical spectra [10] of the biological tissues alone.